In many applications an electronic chip is provided to perform one or more functions, including the control of current provided to an external device, or load. In order to ensure proper powering of the external device, or load, the provided current should be accurately measured and/or controlled. For example, Power over Ethernet (PoE), in accordance with both IEEE 802.3af-2003 and IEEE 802.3at-2009, each published by the Institute of Electrical and Electronics Engineers, Inc., New York, the entire contents of each of which is incorporated herein by reference, defines delivery of power over a set of 2 twisted wire pairs without disturbing data communication. The aforementioned standards particularly provide for a power sourcing equipment (PSE) and one or more powered devices (PD). In order to properly power the PD, and avoid overload in the case of short circuits, the current output by the PSE should be controlled so as not to exceed a predetermined limit. Additionally, the current output by the PSE should be accurately measured in order to determine if there is enough power for all of the PDs.
Prior art methods of measuring and closed-loop controlling of a current include measuring a voltage representation of the current across an external sense resistor, however this requires extra input/output pins and external connections, thus increasing cost. An internal sense resistor would thus be preferable, however unfortunately on-chip resistors exhibit only an approximately known resistance with a large tolerance, and as a result accurate measurement and adjustment of the current cannot be performed using an on-chip resistor without expensive trimming or calibration. While a wide tolerance is typical for an individual resistor, the relationship between the various resistors in a region of an integrated circuit can be well controlled.
There is thus a long felt need for a way to accurately measure and control the current provided by an integrated circuit chip utilizing an on-chip resistor without requiring expensive trimming or calibration.